The present invention relates to pulsed-current arc welding processes and apparatuses, and more particularly to gas-shielded pulsed-current arc welding processes and apparatuses using consumable wire.
In general, gas-shielded arc welding processes using consumable wire consist in striking an electric arc between the sheets to be welded and a consumable welding wire. The consumable wire undergoing a translational movement is progressively melted by the arc and thus contributes to the formation of a weld bead.
The fact of being able to feed the wire at different speeds gives rise to the existence of several modes of operation, the welding characteristics of which are completely different. Depending on the value of the current and of the voltage, the way in which the molten metal is transferred from the wire to the liquid pool varies considerably.
At low currents (80 to 150 A), the Joule effect and the heat released by the arc melt the end of the wire, thus forming a drop of molten metal. Since the wire continues to advance, the drop comes into contact with the weld pool. There is then a short circuit (extinction of the arc) and it is at that moment that the drop is detached, and then transferred into the pool. When the transfer has taken place, the arc is reignited and the process may thus start all over again. This constitutes the short-circuit transfer (short arc) mode.
At higher currents (150 to 250 A), the transfer mode is globular. This transfer mode is not used very much in practice since it results in erratic transfer and in considerable spatter. In this mode, the drop may grow inordinately and either explode before any contact with the sheet or be transferred by short-circuiting incorrectly, often outside the pool.
At very high currents and about 30 to 35 V, depending on the shielding gas used, the welding enters the axial spray (spray-arc) mode in which a string of small drops of molten metal regularly escape from the end of the molten wire.
The current in this case is high enough to expel the drops formed before any contact with the sheet.
There is a fourth, more artificial, transfer mode which, from the standpoint of the average welding energy, lies within the globular range: this is the pulsed mode. In this case, welding is carried out using a pulsed current, the pulse parameters being chosen in such a way that, for each pulse, transfer is of the spray-arc type with a single drop per pulse. This requires special generators in which the current waveform is controlled. This mode is often used as it allows good transfer within an average energy range in which conventional DC welding is difficult to put into practice. However, this mode is very noisy and it is not unusual to reach levels of acoustic noise close to 100 dBA (at 40 cm from the arc), which quite easily exceed the thresholds defined in the legislation concerning the regulation of noise emitted by machines. In pulsed mode, the pulse may have various waveforms: the trapezoidal waveform is most used in practice, but it is possible, by means of transistorized current transformers, to give the pulse various waveforms, while still controlling quite well the parameters which define it.
The procedure for establishing a pulsed-current welding programme consists in finding the most suitable situation between the pulse parameter values and the desired welding result. That is to say, for each wire, shielding gas and wire feed speed, it is necessary to determine the values of these parameters which best satisfy the criteria according to which the welding operation is judged. In general, a good pulsed-current welding programme must be one in which only one drop is detached per pulse, with drop diameters of about the diameter of the wire. For a given wire feed speed, this therefore determines a pulse frequency. Moreover, the common belief is that the rise and fall slopes of the current should be as high as possible so as to obtain stiff and highly directional arcs which, for most operators, are the best way of producing a weld bead in pulsed mode. The other parameters are generally chosen empirically.
Thus, the document JP-A-57,124,572 describes a pulsed arc welding process using a consumable electrode and a shielding gas consisting of an inert gas. According to this document, the amplitude of the current must be at least 300 A, and the duration extending between the start of the current increase and the start of the current fall, that is to say including the peak time during which the current is at its maximum value, is between 0.2 ms and 4 ms, this being the case for triangular or rectangular pulses.
However, the problem of noise pollution caused during an arc welding operation in pulsed mode has not hitherto been solved.
The object of the invention is therefore to provide an arc welding process in pulsed mode having an acceptable level of acoustic noise (of less than approximately 90 dBA) and good welding characteristics.
For this purpose, the subject of the invention is a pulsed-current arc welding process, characterized in that each electric current pulse has rise and/or fall slopes of between 50 A/ms and 1000 A/ms, apart from triangular pulses.
Preferably, each current pulse has a peak time of non-zero duration.
Within the context of the present invention, the expression xe2x80x9cpulse having a peak time of non-zero durationxe2x80x9d should be understood to mean a pulse whose representation, visualization or measurement shows that there is a non-zero duration separating the end of the rise of the current and the start of the fall of the current. Over the period of this non-zero duration of the peak time, the amplitude of the current may, depending on the case, be maintained approximately constant or, in contrast, may undergo slight variations, as will be explained in detail below.
Depending on the case, the process may include one or more of the following characteristics:
the rise and/or fall slopes are between 100 A/ms and 600 A/ms, preferably between 150 A/ms and 500 A/ms;
the arc welding process is a consumable-wire process;
the pulse frequency is adjusted so as to obtain drops of molten metal with a diameter, at the moment of their release, of between 1 and 1.4 times the diameter of the consumable wire;
the pulse frequency is adjusted so as to obtain drops of molten metal with a diameter, at the moment of their release, of approximately 1.2 times the diameter of the consumable wire;
the pulses are trapezoidal;
the pulses are composed of a combination of several identical, similar or different patterns chosen from trapezoidal, sinusoidal, triangular, rectangular and square patterns;
the welding process is carried out under a flow of shielding gas that includes an active gas; and
the active shielding gas comprises argon and/or helium, and at least one compound chosen from CO2, H2, O2 and mixtures thereof.
The present invention furthermore relates to a welding apparatus capable of implementing the process according to the invention and, in particular, to a pulsed-current arc welding apparatus that includes means allowing electric current pulses to be obtained which have rise and/or fall slopes of between 50 A/ms and 1000 A/ms, apart from triangular pulses.
Preferably, the apparatus of the invention allows pulses to be obtained which furthermore have a peak time of non-zero duration.